Due to the limitations of chip area and power consumption, a mobile device may tend to adopt a digital circuit that integrates all components of a computer or other functional circuits into a single chip. The digital circuit may also comprise power switches to divide the functional circuits into several different power islands. The power supplies of the power islands are independent of each other. Namely, each of the functional circuits in the digital circuit is connected to the power supply through its power switches so the connection between the power supply and a functional circuit can be shut down by turning off the power switches when the functional circuit is idle. Thus, the power island divisions can help the digital circuit to save power consumption.
As the operation frequency of the mobile device increases, the functional circuits in the mobile device may influence each other, especially functional circuits that are processing radio-frequency (RF) signals, including a global positioning system (GPS), wireless fidelity (Wi-Fi), Bluetooth (BT) and frequency modulation (FM). These radio-frequency functions involve high frequency analog signals and high speed digital operations that consume huge digital currents and induce lots of switching noises. The high frequency spurs generated by the switching noises may fall into the interested RF band and couple to the RF receivers through the package or substrate causing sensitivity degradation of the RF receivers. The sensitivity degradation may get even worse when the chip size becomes smaller and the digital current becomes larger.
To reduce the mutual influences between the functional circuits, including the sensitivity degradations on functional circuits processing RF signals, extra circuits may be required to compensate the high frequency spurs. However, it could be very difficult to further embed more circuits into the digital circuit since the chip size has been strictly limited for mobile devices. Also, the current consumed by the functional circuits may vary with different operational modes so the high frequency spurs may also be changed, which makes it even harder to solve the issue of sensitivity degradation for functional circuits processing RF signals. Therefore, a solution without increasing the chip area is needed.